String Theory/Holography/Gravity

Quantum physics and Einstein's theory of general relativity are the
two solid pillars that underlie much of modern physics. Understanding
how these two well-established theories are related remains a central
open question in theoretical physics. Over the last two decades,
efforts in this direction have led to a broad range of new physical
ideas and mathematical tools. These have deepened our understanding not
only of quantum gravity, cosmology, and particle physics, but also of
intermediate scale physics, such as condensed matter systems, the
quark-gluon plasma, and disordered systems. Ideas from string
theory have also led to new insights and approaches to problems in many
areas of mathematics. Indeed, the interface of quantum physics and
gravity is a vibrant area of research that is expected to be extremely
active in the coming decade.
Researchers in the CTP have been at the forefront of many of
these developments. CTP faculty members work on string
theory foundations, the range of solutions of the theory, quantum
cosmology, and the application of string-inspired ``holographic''
methods to strongly coupled field theories. The group in
the CTP has close connections to condensed matter
physicists, astrophysicists, and mathematicians both at MIT and
other departments.

Even though we understand string theory better, there is still no clear
fundamental description of the theory in a background-independent
framework, and the set of solutions, or string vacua, is still poorly
understood. The work of Washington
Taylor and Barton
Zwiebach combines physical insight with mathematical consistency
to address these questions, and has led to the development of new
mathematical results and ideas.

Taylor's recent work has given a new systematic understanding of
certain types of complex manifolds used for string compactifications,
and has identified specific constraints and generic properties that
large classes of string theory solutions imply for quantum gravity
theories in four and higher dimensions. This program has led to
evidence that with additional dimensions and supersymmetry, the
spectrum of any consistent quantum gravity theory must arise from a
solution of string theory.

Zwiebach has been at the forefront of developments in double-field
theory. This program has identified structures of generalized
geometry that are relevant to the description of gravity in string
theory and, despite advances, is still in its infancy.

Outside the direct application to questions of quantum gravity,
string theory has in recent years spawned a rapidly-developing new
area of application where "holographic" dualities relate theories of
gravity in one spacetime to strongly-coupled quantum theories in a
spacetime of one less dimension. Originally discovered in the context
of string theory, these dualities appear to provide a rigorous
mathematical equivalence between the two related theories. Such
dualities give both a new perspective into quantum gravitational
phenomena as encoded in quantum field theory, and a way to
explore aspects of strongly coupled field theories using the
gravitational dual. CTP faculty have played a pioneering role in
several applications of holographic duality. Hong Liu and Krishna
Rajagopal were at the forefront of efforts that used holography to
find new insights into the physics of the quark-gluon plasma. Liu
and (ex-CTP faculty member) John McGreevy pioneered the use of
holographic methods to study strongly-coupled condensed matter
systems (now known as "AdS/CMT" duality). More recently, Allan
Adams and Liu have used holographic methods to study turbulent
flows in both ordinary fluids and superfluids. Adams has also
initiated the use of holography for studying disordered systems. In
all these cases, the holographic approach provides qualitative insight
into the physical behavior of systems that are difficult to understand
directly using conventional methods.

In recent years, a set of interesting new developments has begun
to draw unexpected connections between a number of problems
relating aspects of gravity, black holes, quantum information, and
condensed matter systems. In particular, it is becoming clear that
quantum entanglement, through holographic duality, underlies a
characterization of spacetime geometry. These developments tie into
the research activity of several CTP faculty members, including Aram
Harrow and Hong Liu.

The string group in the CTP interacts broadly with the other groups
within the CTP, and with the astrophysics group in the physics
department. Faculty in other departments working in string-related
areas include Isadore Singer (math). In addition to the regular MIT
faculty, Ashoke Sen spends two months each year with the group as
the Morningstar visiting professor.